1. Field of the Invention
The present invention relates to a cap member for covering a sensor unit and to a sensor device including such a cap member.
2. Description of Related Art
Some examples of a sensor device having a sensor unit contained in a cap member are disclosed in JP-A-2006-112801 and JP-A-2007-147461. The sensor unit includes a magnetoresistive element (MRE) and a magnet for giving a biasing magnetic field to the MRE. A rotational speed is detected based on resistance changes in the MRE due to a magnetic field intensity. The cap member is formed by molding a resin material into a cylindrical shape having a bottom end wall. A sensor chip in a bare chip state is used in the sensor unit, and an opening of the cap member is closed by laser-welding a closing member.
The cylindrical resin cap member having a bottom end wall is molded in a molding die including an outer die and an inner die. The inner die has to be separated from the cap member in its axial direction after molding operation is completed. In the process of separating the inner die from the molded cap member, there is a tendency that molded cap member adheres to the inner die and the molded cap member is taken out from the outer die together with the inner die. To avoid this problem, it is conceivable to provide a certain taper to an outer surface of the inner die. However, this is not easy from a standpoint of a shape of the biasing magnet to be contained in the cap member.
It is also conceivable to push the bottom wall of the cap member with an ejector pin to forcibly separate the inner die from the cap member. However, the bottom wall or other portions may be deformed by the ejector pin because a thickness of the bottom wall is made thin to obtain a high sensitivity of the sensor device. Separation of the inner die from the cap member becomes more difficult when a molding material having high molecule numbers, a lower content of glass-additives and a lower content of die-separating agents. This type of the molding material is used to improve transparency of laser in a process of welding a closing member to the cap member by laser. By reducing the die-separating agents, separation of the inner die becomes more difficult. By using a molding material having higher molecule numbers and a lower content of glass-additives, a curing time of the molded cap member becomes longer. This makes a cycle time of injection molding longer.